A diffraction grating is an optical element that consists of a reflecting or transparent substrate whose surface contains fine grooves or rulings that are equally spaced. When light is incident on a diffraction grating, diffractive and mutual interference effects occur and light is reflected or transmitted in discrete directions called orders. Because of their dispersive properties, gratings are commonly used in monochromators and spectrometers. These devices were first manufactured by German physicist Joseph von Fraunhofer in 1821.
Diffraction gratings have been a research topic for many decades. At the beginning, the design and the functional principle of the gratings were of major interest. In recent years, tunable diffraction gratings became popular, which allow to modify their physical behavior. One of the major topics was how a large tuning range could be achieved. The intense research activities combined with new micromachining technologies resulted in several types of tunable diffraction gratings. Examples of these tunable diffraction gratings are different realizations of mechanically rotary gratings or tunable gratings based on comb drives (see A. Azzam Yasseen et al. “Diffraction Grating Scanners Using Polysilicon Micro-motors”, IEEE Journal of selected topics in quantum electronics, Vol. 5, No. 1, January/February 1999). Although some of these gratings are employed in commercially successful products, they are often limited in their tuning range or require many expensive production steps.
Classical diffraction gratings are normally based on non-deformable rigid materials. These materials make it impossible to change the shape of the gratings. Therefore the only possibility to tune these gratings is rotation. As state of the art solutions show, the rotation of gratings involves expensive mechanical systems that are difficult to implement in a micro system. In recent years this problem was addressed and several attempts to solve it were undertaken. Tunable diffraction gratings based on comb drives or piezoelectric actuators were developed. Such devices require either high investments due to complicated production processes or achieve only relatively small tuning ranges. This is mainly due to the stiff materials that are used for the implementation of the grating.
Tunable diffraction gratings are known from prior art. One example of such a device is a diffraction grating which is based on polymer substrates (i.e. PDMS) and which changes its shape due to thermal expansion (see Bartosz A. Grzybowski et al., “Thermally actuated interferometric sensors based on the thermal expansion of transparent elastomeric media”, Review of Scientific Instruments, Vol. 70, No. 4, April 1999). The same researchers have developed another tunable diffraction grating, which is tuned by the application of an external mechanical pressure (see Bartosz A. Grzybowski et al., “Beam redirection and frequency filtering with transparent elastomeric diffractive elements”, Applied Optics, Vol. 38, No. 14, 10 May 1999). The mechanical pressure is applied to a layer of polymer comprising a diffraction grating by two parallel glass plates.
Due to the function principle both solutions have the disadvantage that it is e.g. not possible to miniaturize them.
US2004/0109234 (US'234), by Tapani Levola, was published in 2004 and relates to an optical device for manipulating a light wave using a diffractive grating structure which is electrically deformable. The electrically tunable diffraction grating is based on the physical effect that at an interface of two materials with different dielectric constants a force occurs in the presence of an electric field. This effect is suggested to be used to tune the diffractive behavior of the grating.
US2002/0186928 (US'928), resp. EP147373, by Curtis Kevin, was published in 2002 and describes a tunable optical device for adding or dropping one or more channels in a wavelength division multiplexing communication system. The tunable optical device comprises one or more filters, wherein at least one filter comprises at least one elastomer and one or more gratings. The elastomer is a polymer that expands and contracts with a change in a voltage applied across the polymer or when a certain wavelength of light is diffracted from or transmitted through the polymer. This device consists of a holographic grating that is based on an alternating refractive index pattern. This grating is embodied within an elastomer that changes its thickness when a voltage is applied. One disadvantage is that the grating is based on a fixed pattern of alternating refractive indices.
U.S. Pat. No. 6,930,817 (US'817), by Srinivasan et al., was published in 2004 and discloses a variable modulator assembly which includes an active layer with a multiplicity of electrodes. A deformable layer is in operational contact to a first surface of the active layer. An electrode configuration with a plurality of electrodes is in operational contact to a second surface of the active layer. A controller is configured to selectively apply a variable signal to the selected electrodes of the electrode configuration. Application of the variable signal causes the deformable layer to reconfigure to an alternated shape having distinct peaks and valleys. The distance between the peaks and valleys being determined by the value of the applied variable signal. In an optical modulating method, a variable modulator assembly is positioned to receive light at the deformable layer from a light source. Activation of an electrode configuration by the controller generates a variable signal, causing electrostatic charges to distort the deformable layer into a pattern corresponding to the activated electrodes.
U.S. Pat. No. 6,903,872 (US'872), by Schrader, was published in 2002 and describes an electrically reconfigurable optical device based on the use of a layer of dielectric and transparent viscoelastic material opposing at least a first electrode structure. According to the invention the arrangement of the individual electrode zones in the first electrode structure in order to deform the viscoelastic layer complies with one of the following alternatives. According to the first alternative, the electrode zones of the first electrode structure are grouped into groups composed of two or more adjacent electrode zones and within each of said groups individual electrode zones are supplied each with a substantially different voltage. According to the second alternative, the electrode zones of the first electrode structure are substantially annular, elliptical, rectangular or polygonal closed-loop electrodes. The invention allows, for example, for creating electrically reconfigurable blazed gratings or Fresnel zone lenses.
U.S. Pat. No. 3,942,048 (US'048), by Laude et al., was published in 1976 and is directed to an optical grating assembly which comprises a piezo-electric substrate. The piezo electric substrate supports on two opposite faces metallic layers. One of these faces of the substrate also carries a grating either formed in that face, in the metal layer supported by that face, or in a resin layer carried by that face. Application of a variable voltage between the metal layers sets up an electric field of variable strength in the substrate and this results in the pitch of the grating being variable due to the piezo-electric nature of the substrate. Due to the fact that this grating relies on the piezo electric effect of the substrate only limited pitch variation is possible and therefore no large tuning range results. A further difficulty consists in that the making of such a device is relatively expensive.
U.S. Pat. No. 4,850,682 (US'682), by Gerritsen, was published in 1989 and describes a diffraction grating. The diffraction grating responds to radiation incident thereon within a given range of incidence angles and redirects the incident radiation from the structure in a selected direction within relatively limited confines. A liquid crystal material is positioned in contact with the diffracting surface of at least one diffraction structure. When inactivated, the liquid crystal material has a refractive index which is substantially the same as that of the diffraction structure. When activated the refractive index of the liquid crystal material is substantially different whereby incoming radiation within a given range of incidence angles is transmitted through the structure and exits in the selected direction.
WO9948197 (WO'197), of Trex Communications Corp., was published in 1999 and describes a piezoelectric substrate having an attached or integrally formed diffraction grating. When an electric field is applied to the piezoelectric substrate parallel to the grating, the piezoelectric actuator stretches the grating such that the periodicity and the angle of diffraction changes. One problem consists in that the electrodes are arranged lateral to the grating. Thereby it becomes not possible to miniaturize the device.
WO2005085930 (WO'930), of Siemens Aktiengesellschaft, was published in 2005 and is directed to an adaptive optical element which can be configured e.g. as a biconvex lens. The element has a polymer actuator which is constituted of an electroactive polymer layer and several lateral layer electrodes. The layer electrodes are exposed to different voltages, thereby producing a gradient in the field strength of the electric field influencing the deformation of the polymer layer. It is described that it would be possible to achieve almost any state of deformation such as e.g. the depicted biconvex lens. However, the lens does not seem to be very accurate.
In WO'930 reference is made to an article published in “Smart structures and materials 2001”, Vol. 4329 which describes a membrane actuator comprising an electroactive polymer which is arranged within an opening of a circular frame. In that a voltage is applied to electrodes arranged on opposite sided of the membrane the surface of the membrane can be modified. Due to the reason that the membrane is attached in a slack manner in the frame the membrane starts to sag when it's surface is increased due to a voltage applied. Thereby it is possible to take influence on a beam traveling through the membrane.
The above mentioned prior art devices are not only expensive in production but also have the disadvantage of not being practical in the transmission mode. However, the device known from US'928 could be used as a transmission grating in certain applications but only as a wavelength filter and not as a light steering device.